Use conditional mutations to update metadata table

Details

Description

For correctness Accumulo requires that only one tablet server at a time serve a tablet. In order to enforce this constraint, Accumulo uses zookeeper locks. It's assumed when a tablet server lock disappears that the tablet server will kill itself. Therefore a tablet that's assigned to a dead tablet server can be safely reassigned. However sometimes tablet servers continue to operate for a period of time after losing their locks. Sometimes this is caused by bugs in Accumulo, sometimes it's the Java GC or swapping (and the tserver does die), sometimes it's problems with zookeeper (like the zk thread that reports lock lost dies).

In Accumulo 1.6 conditional mutations were added. Making all tablet metadata updates use conditional mutations could make multiply-assigned tablets less able to do damage.

For example if after a minor compaction, the metadata update mutation could require the tablet location to be the current tserver: it would prevent a zombie tserver from adding an extraneous file to the metadata table for a tablet.

Christopher Tubbs has discussed refactoring all metadata code so that its more modular and works with zookeeper (for root tablet) and metadata table using same API. This solution could depend on that. It may also be useful to make the root tablet operate more like a regular tablet and store its list of files in zookeeper. Then the root tablet could benefit from these changes with the right abstraction layer.

Activity

For reference, the rudimentary framework for the refactoring that is mentioned is located in o.a.a.core.metadata.MetadataServicer. It doesn't do much now, but the ideas was to move stuff in to clean up client code (TabletLocationObtainer, for example) and other metadata operations.

Christopher Tubbs
added a comment - 28/Jan/14 23:18 For reference, the rudimentary framework for the refactoring that is mentioned is located in o.a.a.core.metadata.MetadataServicer. It doesn't do much now, but the ideas was to move stuff in to clean up client code (TabletLocationObtainer, for example) and other metadata operations.